setting up shop page with allproducts widget & category menu

I’m trying to set up a website for my wife’s and my antiques/collectibles sales. We’re retired and the money we make supplements our retirement – enough to do a bit of traveling. I created a website in Joomla a number of years ago but was never really happy with it and with recent updates to Joomla, it is now just an error message. I’ve also done a few sites in html and also drupal so I’m not a neophyte. But this is the first time I’ve tried WordPress. I’m well into my 70s and maybe the brain cells aren’t ticking over as fast as they used to, but with a little help I’m confident it’s doable. I’m not against spending a little money on enhancements but i want to get the basic setup working before I invest anything other than time. For the moment I’m doing this in Xammp.
Everything in the antique business is unique with an inventory of one. I want to create a site that shows as many items as possible (up to a thousand items) and? have them searchable and displayed by category, keywords, etc. I’ve enabled WooCommerce and was looking at the Storefront theme but I decided that the Astra theme was a bit more flexible. I also like Spectra.

So this is the issue: I want to use the Allproducts widget (woocommerce) to display the items (I’m thinking 4 items wide by 10 or so rows) and I want a sidebar with a menu of categories (there will be 20 or 30 of them). I can set up a sidebar column with the menu and put the Allproducts widget in the other, but I want to make the sidebar narrower. I found some CSS to do this but when I try to edit the page the sidebar disappears. How do I edit it when it isn’t there?

Once I get past that I’m sure there will be other issues. I hope you can help.

The page I need help with: [log in to see the link]